The present disclosure relates generally to polycarbonate nanocomposites prepared by polymerizing sulfonated telechelic polycarbonates in the presence of organically modified clays. Processes for producing such nanocomposites, as well as articles formed from the same, are also disclosed.
Nanocomposites are particle-filled polymers for which at least one of the dimensions of the dispersed phase is in the nanometer (10−9 meter) or nanoscale range (typically from about 1 to about 20 nanometers). Nanocomposites often have superior physical and mechanical properties over their microcomposite counterparts, such as improved modulus, reduced gas permeability, flame retardance, and improved scratch resistance. Moreover, the nanoscale dispersion of the filler does not give rise to the brittleness and opacity typical of microcomposites or larger sized fillers.
Clay-based nanocomposites are obtained by the admixing of extraneous materials, such as polymers, with layered clay. Layered clay consists of metal silicates that are arranged in layered structures which are stacked in an orderly fashion. Admixing breaks the ordered layering of the clay into smaller particles, which are also known as platelets. The platelets have the improved properties described above. The clay is either blended with the polymer or blended with a monomer followed by in situ polymerization.
Polycarbonate nanocomposites have not been extensively researched compared to other types of nanocomposites. This may be because of the poor mechanical and color properties of the polycarbonate nanocomposites which have been obtained so far. D. R. Paul reported in Polymer 2003, vol. 44, pp. 5323-5339, that only a small fraction of the clay platelets are exfoliated while the main part of the polycarbonate/clay composite has an intercalated morphology. The reason for this low degree of dispersion may lie in the poor compatibility between the ionic clay surface and the non-polar polymer. As a result of this low degree of dispersion, the polycarbonate nanocomposite has inferior optical properties such as, for example, reduced transparency or increased haze.
D. R. Paul also reported in Polymer 2003, vol. 44, pp. 5341-5354, that the polycarbonate/clay composite is generally dark colored and that the weight average molecular weight (Mw) of the polycarbonate matrix consistently drops by 30% to 40% after extrusion from the melt process used to combine the materials. Typically carried out at about 300° C., the drop in Mw suggests that the polycarbonate matrix may not be thermally stable.
Furthermore, the color of the nanocomposite may depend on the type and purity of the clay and on the surfactant used to modify the clay. Ammonium surfactants, commonly used to modify clays during the manufacture of nanocomposites, cannot be used for polycarbonate nanocomposites prepared by melt methods since the thermal stability of the resulting clays is below normal processing temperatures for polycarbonate (280 to 320° C.). This leads to the formation of degradation products, providing mechanisms for the consistent decrease in Mw and strong discoloration of the product.
There remains a need for methods that reduce the degradation of the polymer matrix and increase the degree of dispersion of the clay. There is also a need for polycarbonate nanocomposites having improved thermo-mechanical properties and better color.